Problem: Solve for $k$ : $3 = -9 + k$
Answer: Add $9$ to both sides: $ \begin{eqnarray} \\ \hphantom{3 {+ 9}} &\hphantom{=}& \hphantom{k} \\ 3 &=& -9 + k \\ \\ {+9} && {+9} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 3 {+ 9} &=& k \\ \end{eqnarray} $ Simplify. $ k = 12$